// 修改数组
let arryCopy = [1,2,3,4,5].copyWithin(0,3);
console.log(arryCopy);

//==== 不理解
let arryfu = [1,2,3,4,5].copyWithin(0,-2,-1);
console.log(arryfu); // [4, 2, 3, 4, 5]

// ...